1. Field of the Invention
The present invention relates to a method and system for controlling power distribution in a hybrid fuel cell vehicle.
2. Background Art
A hybrid fuel cell vehicle may include three power sources for its electrical loads: a battery, the fuel cell, and a traction motor. While powering the vehicle, the traction motor is a load, but during coast-down, the traction motor becomes a generator. This regenerative power can supply current to the other loads or be used to charge the battery. Coordinating the current flow between these sources and the continually varying electrical loads presents a fundamental control problem. Imprecise control can result in reduced fuel economy, poor performance, reliability problems, and possible electrical bus instabilities.
In addition, there are other considerations when utilizing a fuel cell in a power distribution system. For example, unlike a battery, a fuel cell may not be able to instantly supply sufficient current to meet the needs of an increased electrical load.
Therefore, the battery needs to xe2x80x9cfill inxe2x80x9d current temporarily, then taper off when the fuel cell""s current output increases. Without the presence of the battery to temporarily supply current, performance may degrade. In addition, the battery may also provide a repository for excess fuel cell current and regenerative current during braking and coast down.
One attempt to integrate a battery into a hybrid fuel cell vehicle is described in SAE Paper No. 2002-01-0096, titled xe2x80x9cDevelopment of Fuel-Cell Hybrid Vehiclexe2x80x9d (xe2x80x9cthe SAE Paperxe2x80x9d). The SAE Paper describes the use of a battery connected in parallel with fuel cells via a DC/DC converter. The battery is configured to provide a power assist when fuel cell response is delayed, or when the vehicle is driven under high load conditions. The traction motor is located between the fuel cell and the converter; whereas, the fuel cell auxiliary systems are located between the battery and the converter. To determine the fuel cell operational point, power-current (P-I) and current-voltage (I-V) maps are used. A power requirement is input, and using the P-I and I-V maps, a voltage command is determined.
One limitation of the hybrid vehicle described in the SAE Paper is its use of operating modes which do not utilize the fuel cell, but rather, rely solely on the battery to supply all of the power. In such operating modes, all of the vehicle electrical loads are carried by the battery. This may require the use of an undesirably large battery, or place limits on the loads the system is able to handle.
In addition, the SAE Paper does not describe a system or method for controlling the rate of change of current flow to or from the battery, nor does It describe how to determine a target rate.
Accordingly, there exists a need for a method and system that provide for controlling power distribution in a hybrid fuel cell vehicle such that a fuel cell works in conjunction with a second power source, such as a battery, ultra-capacitor, or other equivalent electrical storage device, to provide power to vehicle electrical loads, and a system equilibrium is sought, wherein the fuel cell carries all of the vehicle electrical loads, and the current flow of the second power source is adjusted at least partly based on a measured voltage, and at a predetermined rate, until a predetermined constant is reached.
Therefore, a power distribution control system for a vehicle having a fuel cell and a second power source connected to an electrical bus is provided. The control system includes a voltage regulator configured to control voltage on the bus. A first controller controls the voltage regulator. A computer is programmed and configured with fuel cell characteristics for relating fuel cell voltage to fuel cell current. The computer is further programmed and configured to receive a current request at least partly based on vehicle loads, and to determine a first voltage related to the current request using the fuel cell characteristics. A second controller is configured to receive a voltage signal from the computer and to provide a current command to the first controller. The voltage signal is at least partly based on the first voltage and a measured voltage.
Some embodiments of the invention also include a power distribution control system having electrical loads connected directly to the fuel cell, which provides a low cost, efficient architecture. Since main power current can go directly from the fuel cell to the loads without passing through another device, the battery and voltage regulator size can be minimal. This may result in an overall cost savings.
In addition, embodiments of the invention may utilize a single voltage sensor to measure a voltage on the electrical bus to help control the voltage regulator. Because voltage sensors are often used to provide voltage measurements to other vehicle systems, a separate voltage sensor may not be needed in the present invention. Moreover, the use of a single voltage sensor, rather than multiple sensors, may provide an overall cost savings.
The invention also provides a method of controlling the power distribution in a vehicle having a fuel cell and a second power source. The method includes generating a first voltage based on a vehicle electrical load change. A first current command is generated at least partly based on the first voltage and a measured voltage. Current flow of the second power source is adjusted at least partly based on the first current command, and the available fuel cell current is adjusted at least partly based on the vehicle electrical load change. The current flow of the second power source is continuously adjusted at least partly based on additional current commands until an equilibrium point is reached.
The invention further provides a vehicle having a fuel cell and a second power source connected to an electrical bus, and a power distribution system for controlling the distribution of power in the vehicle. The power distribution system includes a voltage regulator configured to control the voltage on the bus. A first controller controls the voltage regulator, and a computer is programmed and configured with fuel cell characteristics for relating fuel cell voltage to fuel cell current. The computer is further programmed and configured to receive a current request at least partly based on vehicle loads. The computer is also programmed and configured to determine a first voltage related to the current request using the fuel cell characteristics. A second controller is configured to receive a voltage signal from the computer, and to provide a current command to the first controller. The voltage signal is at least partly based on the first voltage signal and a measured voltage.
The invention also provides a controller for controlling the power distribution in a vehicle having a fuel cell and a second power source. The controller includes an algorithm for generating a first voltage at least partly based on vehicle electrical loads, for generating a current command at least partly based on the first voltage and a measured voltage, for adjusting current flow of the second power source at least partly based on the current command, for adjusting available fuel cell current at least partly based on the vehicle electrical loads, and for continuously adjusting the current flow of the second power source until an equilibrium point is reached.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings.